Comet Hartley 2 Fires Out Poison Gas as NASA Probe Nears

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As a NASA spacecraft speeds toward its rendezvous tomorrow (Nov.
4) with Comet Hartley 2, the icy wanderer is putting on a show,
blasting out gassy jets and coughing up loads of poisonous
cyanide.

In less than 24 hours, the Deep Impact probe will cruise to
within 435 miles (700 kilometers) of Hartley 2, marking just the
fifth time that a comet has ever been observed up close. And the
view should be interesting.

Last week, the spacecraft spotted two massive jets spewing from
the comet's nucleus its core of ice and rock.
Comet Hartley 2 also spat out huge quantities of cyanide over
a week-long stretch in September, raising concentrations of the
poisonous gas by a factor of five, researchers said. [ Video
of Comet Hartley 2 jets.]

"We're about to be surprised," said Mike A'Hearn of the
University of Maryland, principal investigator of Deep Impact's
mission, which NASA calls EPOXI. "This comet is unlike any we've
visited before, and we don't know what we're going to find."

The encounter: How it should happen

Comet Hartley 2 is a small, active comet that orbits the sun once
every 6 1/2 years. It was discovered in 1986 by Australian
astronomer Malcolm Hartley. [ Q&A
with Comet Hartley 2 Discoverer ]

The Deep Impact spacecraft has been chasing Hartley 2 for months,
taking pictures and maneuvering to prepare for the close flyby,
which is scheduled to take place at 10:01 a.m. EDT (1601 GMT) on
Nov. 4.

Here's a brief rundown of how the flyby should proceed, if
everything goes according to plan:

Later this evening (Nov. 3), about 18 hours before its closest
approach, Deep Impact will begin the encounter phase of its
mission. The probe will reorient itself so its three imagers two
in the visible-light spectrum and one in the infrared can lock on
Comet Hartley 2 for the next 24 hours-plus.

This maneuver will point the probe's high-gain antenna away from
Earth, meaning Deep Impact won't be able to beam pictures home
for a bit, researchers said. Instead, it will store the photos
onboard in its two computers.

This lack of multitasking ability reflects the fact that Deep
Impact is a recycled, repurposed spacecraft. It was originally
designed to serve as a mother ship for NASA's Deep Impact
mission, which intentionally
crashed a probe into Comet Tempel 1 in 2005 to study the
object's composition.

Soon after Deep Impact's closest approach with Hartley 2, mission
scientists will reorient the probe so it can both track the
now-receding comet and start beaming images to Earth.

The spacecraft will continue to take new photos while it sends
its cache of comet close-ups home. The first few photos should
reach researchers' computers about an hour or so after the
rendezvous, scientists said. The complete data dump, however,
will take a while.

Researchers should get some good looks at Comet Hartley 2. One of
Deep Impact's cameras is so powerful that it can distinguish the
size difference between a car and a pickup truck from 400 miles
(645 km) away, mission scientists have said.

Data from the close approach will continue to download through
Saturday (Nov. 6) but NASA will release preliminary results
sooner than that. A news conference is scheduled for 4 p.m. EDT
(2200 GMT) tomorrow (Nov. 4), just six hours after Deep Impact's
closest approach to Hartley 2, agency officials said.

Post-encounter: Watching Hartley 2 speed away

Though Deep Impact will zoom by Comet Hartley 2 incredibly fast
during the flyby at about 27,000 mph (43,548 kph) the spacecraft
will be able to track the comet's tail end with just a slow
swivel. The probe will turn about 1 degree per second, according
to mission scientists one-sixth the rate at which a second hand
makes its way around a clock face.

This anticipated ease of tracking is one reason mission planners
chose to stay 700 kilometers away from the comet, rather than zip
in for a closer, more detailed look.

Another reason: to keep Deep Impact away from the densest parts
of Hartley 2's coma, the gassy, dusty cloud around its nucleus.
Particles in the coma could damage Deep Impact's antenna if the
probe gets too close.

Deep Impact will continue photographing Hartley 2 for about three
weeks as the comet speeds off into the dark reaches of space,
Larson added. After that point, the spacecraft's comet-watching
mission will be basically over.

What Comet Hartley 2 can reveal

Researchers hope Deep Impact's flyby past Hartley 2 can give them
a good idea of the composition of the comet's icy nucleus. They
are also eager to compare Hartley 2 to the four other comets that
spacecraft have visited in the past.

Hartley 2, while just under a mile (1.5 km) across, is incredibly
active. A close study of its spewing jets and gas clouds could
yield clues about comet composition and behavior in general,
researchers said.

"We saw outbursts in Comet Tempel 1, as well as many other
things," A'hearn said. "We're trying to find out, are all of the
new phenomena we saw at Tempel 1 universal across all comets, or
are they special to Comet Tempel 1?"

Comets are leftovers from the
solar system's formation 4.5 billion years ago, so studying
them should give astronomers a better handle on how our cosmic
neighborhood came to be, researchers said.